Rotating magnetic disk data storage devices are known in the art. In these devices, one or more read/write heads, typically inductive heads, are used to store data and read data from an associated disk media surface. More specifically, a read/write head is passed over a magnetic medium and transduces the magnetic transitions into pulses of an analog signal that alternates in polarity.
The signals to and from a head-disk assembly of a hard disk drive are then processed mainly by a preamplifier (write driver), i.e., the preamplifier receives from an associated channel device both data signals to be written onto a disk surface during a write operation and control signals used to specify the individual head to be selected for a read or write operation.
Write speeds in hard disk drive preamplifiers are continually improving. An inductive write head includes an inductive coil that can change the localized magnetic fields on the magnetic data-storage medium and thus allows the digital data to be recorded. The speed of this recording process (i.e., the write speed) is determined by how fast the current in a hard disk drive write head can be reversed (the polarity of the write current through the write head being reversed in response to the bit pattern of the information signal). This is also referred to as the "rise-time." Typically the desired requirements for the write driver are a large current capability (e.g., 40-80 ma) combined with a fast rise time (e.g., 1-4 ns) for driving the inductive write head.
The write head for a disk data storage device can be approximately modeled by an inductor with an inductance of L. The voltage across an inductor is ideally proportional to the rate of change of the current through the inductor in time. The mathematical expression for this voltage is given as V.sub.L =L di/dt. Essentially, the voltage across the inductive write head, V.sub.L, is proportional to the value of inductance, L, and to the speed at which the write current is reversed, di/dt. This means that the write current reversal time in inductive write-heads fundamentally depends on how large a voltage can be impressed across the write drive head. Normally, the voltage across the inductor is limited by the supply voltages. Thus, either the head inductance value should be decreased, or, the supply voltage should be increased, to improve the write speed. The first option, decreasing the head inductance value, is normally not preferred, as it negatively affects the reliability of the data-recording process.
Conventional techniques use the power supply to generate the voltage across the inductor. However, the voltage supply limits the voltage that can be applied across the inductor and therefore limits the rise time. Higher write speeds require higher supply voltages. However, the second alternative, increasing the supply voltage, may not always be possible, as system-wide considerations dictate the selection of power supply voltages, and the present trend in fact is the reduction of power supply voltages.
The present invention is therefore directed to the problem of developing a hard disk drive preamplifier that satisfies the demand for improved rise-time while meeting the conflicting demand for maintaining a same supply voltage.